Multifilamentary oxide superconducting wire and method of producing the same, and stranded oxide superconducting wire and method of producing the same

ABSTRACT

A multifilamentary oxide superconducting wire having a low aspect ratio and a relatively high critical current density, and being suitable for a stranded wire, is provided. In order to produce such a wire, a tape-shaped, multifilamentary oxide superconducting wire prepared by the powder-in-tube process is cut along its length. The resulting multifilamentary oxide superconducting wires have a width smaller than that of the tape-shaped wire. The resulting wires typically have an aspect ratio of one to two. The filaments of the resulting wires are in the shape of a ribbon extending in the longitudinal direction, and arranged substantially parallel to each other and in layers. The filaments have an aspect ratio greater than two. The filaments include a filament having a portion substantially not covered with the stabilizer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to multifilamentary oxidesuperconducting wires and methods of producing the same and strandedoxide superconducting wires formed of the multifilamentary wires thatare twined together and methods of producing the same, and particularlyto multifilamentary oxide superconducting wires having a relatively highcritical current density (Jc) while having a relatively low aspect ratioand methods of producing the same and stranded wires formed of suchmultifilamentary wires and methods of producing the same.

[0003] 2. Description of the Related Art

[0004] A multifilamentary oxide superconducting wire produced by a“powder-in-tube” process is expected to be applied to superconductingcables, superconducting equipment and the like with liquid-nitrogencooling. In particular, a tape-shaped multifilamentary superconductingwire having a copper oxide covered with a stabilizing metal isovercoming challenges such as providing a viable critical currentdensity, a viable length, a viable mass-production technique and thelike. Typically, a tape-shaped multifilamentary wire is prepared in thepowder-in-tube process by packing raw material powder for oxidesuperconductor in a silver pipe, drawing the same to prepare amonofilamentary wire, inserting a large number of monofilamentary wiresinto a silver pipe (to prepare a multifilamentary structure), drawingand rolling the multifilamentary structure, and heat-treating themultifilamentary structure to sinter it.

[0005] Because of the crystallographic property of the oxidesuperconductor material, it is advantageous for high Jc that thesuperconducting filament is shaped flat having a high aspect ratio. Sucha flat filament is typically obtained by pressing or rolling. As such, awire of high Jc is provided in the form of a tape having a relativelyhigh aspect ratio. For example, a typical tape-shaped wire has an aspectratio of approximately 20, having a width of approximately 4 mm and athickness of approximately 0.2 mm.

[0006] A tape-shaped wire, however, shows anisotropic properties in thedirection of the magnetic field applied thereto and readily causes an acloss in a fluctuating magnetic field. If a cable conductor is formed ofa set of the tape-shaped wires, unevenness in impedance between thewires can disadvantageously cause a current drift and the like so thatthe conductor can have a significant ac loss. In general, the problemsof current drift and ac loss can rather be overcome by a strandedstructure. It is difficult, however, to form a stranded wire byassembling a large number of tape-shaped wires.

[0007] In contrast, round wires having a round cross section aresuitable for forming a stranded wire. For example, Japanese PatentLaying-Open No. 4-262308 discloses a round wire with metal, silver orsilver alloy and a cylindrical oxide superconductor alternately,concentrically stacked, as seen in cross section. The publicationdiscloses that a multiannular structure having the metal and the oxidesuperconductor alternately stacked is provide, so that the distancebetween the interfaces of the oxide superconductor and the metal can bereduced, more specifically, the adjacent interfaces can be not more than100 μm distant from each other, to provide a c-axis orientation.However, this wire's Jc is a value smaller by one digit than thetape-shaped wire and is not a practically applicable level.

[0008] Japanese Patent Laying-Open No. 5-266726 discloses a method ofproducing an oxide superconducting wire round in cross section with acritical current density hardly depending on the direction of themagnetic field applied. More specifically, the method includes the stepof packing a powdery oxide superconductor in a space formed between ametal tube and a metal core inserted therein, the step of performingplastic working on the tube, and the step of sintering the obtainedwire. This method provides a superconducting wire having a structurewith a cylindrical, oxide superconducting layer between the metal coreand the metal tube. This method would not be suitable for producing awire having a large number of superconducting filaments suitable forcoils or the like.

[0009] Another example of attempting to enhance the critical currentdensity of a round superconducting wire is disclosed in Cryogenics(1992) Vol. 32, No. 11, 940-948. As regards the round wire disclosed inthis literature, 55 monofilamentary rods rectangular in cross sectionare arranged concentrically in three layers in a silver tube. In thereport, the critical current of the obtained wire has not been measured.The wire disclosed in the literature, however, would not have a high Jc.

[0010] U.S. Pat. No. 5,347,085 discloses a multifilamentary oxidesuperconducting wire round in cross section, wherein a plurality offlat, oxide superconducting filaments are arranged in a stabilizer withtheir widths oriented radially. Thus, in the cross section of the wire,the filaments have their widths oriented circumferentially. In general,this wire would hardly provide higher Jc, because such a process of thewire that performs plastic working in the direction of the widths of thefilaments would not enhance a c-axis orientation that can provide a highJc.

[0011] U.S. Pat. No. 5,885,938 discloses a multifilamentary oxidesuperconducting wire having a low aspect ratio in cross section. Thiswire has an oxide superconducting crystal whose c-axis is orientedperpendicular to the longitudinal direction of the wire. In this wire,the filament has an aspect ratio typically equal to that of the wire.This low aspect ratio wire is obtained by performing plastic working ona multifilamentary precursor tape and heat-treating it. In thistechnique, the step of processing a high aspect ratio tape into a lowaspect ratio wire would not provide a high Jc. Thus this technique wouldhardly provide a high Jc wire comparable to the tape-shaped wire.

[0012] Japanese Patent Laying-Open Nos. 9-259660 and 11-39963 disclose amultifilamentary oxide superconducting wire and a method of producingthe same according to the powder-in-tube process. The improved methoddisclosed in the publications includes the steps of: packing an oxidesuperconductor or raw material powder therefor in a tube formed of astabilizing material; performing plastic working on the tube with thepowder packed therein to obtain a tape-shaped wire; packing a pluralityof such tape-shaped wires in a tube formed of a stabilizing material;performing plastic working on the tube with the tape-shaped wires packedtherein to obtain a wire having a substantially round cross section or asubstantially equilaterally polygonal cross section having at least sixangles; and heat-treating the wire to produce a sintered body of theoxide superconductor, wherein the powder portion of the tape-shaped wireto be packed in the tube is in the shape of a ribbon having an aspectratio of 4 to 40 and the wire thermally treated has oxidesuperconducting filaments each having a thickness in a range from 5 μmto 50 μm. In this method, sufficiently compressed tape-shaped wires canbe packed in a tube in an appropriate arrangement to enhance the crystalorientation of the oxide superconducting for a high Jc. However, ahigher Jc is also desired over this technique.

[0013] Japanese Patent Laying-Open No. 6-68727 (filed on Aug. 19, 1992)discloses a production method including the steps of: providing an oxidesuperconducting film on an abraded, long substrate; and cutting thesubstrate to obtain a plurality of superconducting wires. According tothis method, the substrate is cut so as to produce a superconductingwire having a lower aspect ratio than the substrate. In this method,however, it is difficult to obtain a multifilamentary wire.

[0014] Conventionally there is also a technique, as shown in FIG. 1,stacking and bonding a plurality of tape-shaped, oxide superconductingwires 1 together to provide a block 2. In each tape-shaped wire 1, oxidesuperconducting filaments are completely covered with a stabilizingmaterial. Block 2 has a lower aspect ratio than tape-shaped wire 1.Block 2 thus obtained, however, has relatively large width and thicknessso that it is bulky and unsuitable as a strand for use in producing astranded wire. Furthermore, block 2 can contain an excessive amount ofthe stabilizer to provide a Jc per the cross section area of the wire(i.e., overall Jc) reduced.

[0015] D. C. Larbalestier et al., Physica C 221 (1994) 299-303 and G.Grasso et al., Physica C 241 (1995) 45-52 disclose that a relativelyshort, monofilamentary oxide superconducting tape is cut to provide aplurality of pieces thereof for the measurement of the Jc profile in theoxide super conductor. These reports show an interesting result withrespect to a monofilament's Jc profile or variation. However, thesereports only relate to Jc measurement and they never suggests anyinformation useful in processing a multifilamentary wire.

SUMMARY OF THE INVENTION

[0016] One object of the present invention is to provide amultifilamentary oxide superconducting wire having a relatively high Jcand being suitable for a stranded wire and a method of producing thesame.

[0017] Another object of the present invention is to provide a lowaspect ratio, multifilamentary oxide superconducting wire having arelatively high Jc and a method of producing the same.

[0018] Still another object of the present invention is to provide amultifilamentary oxide superconducting wire suitable for a solenoid coiland a method of producing the same.

[0019] Still another object of the present invention is to provide astranded oxide superconducting wire having a relatively high Jc and amethod of producing the same.

[0020] The present invention is directed to a method of producing amultifilamentary oxide superconducting wire, which includes the step ofmaking a cut or cuts in a tape-shaped multifilamentary oxidesuperconducting wire prepared by a powder-in-tube process, along thelongitudinal direction thereof, so that a plurality of multifilamentaryoxide superconducting wires smaller in width than the tape-shaped wireare obtained from the tape-shaped wire.

[0021] Furthermore the present invention is directed a method ofproducing a stranded oxide superconducting wire, which includes thesteps of: making a cut or cuts in a tape-shaped multifilamentary oxidesuperconducting wire prepared by a powder-in-tube process, along thelongitudinal direction thereof, so that a plurality of multifilamentaryoxide superconducting wires smaller in width than the tape-shaped wireare obtained from the tape-shaped wire; and twining a required number ofthe multifilamentary oxide superconducting wires obtained.

[0022] In the present method, preferably, the tape-shaped wire to be cuthas an aspect ratio of at least ten. The obtained multifilamentary oxidesuperconducting wire smaller in width preferably has an aspect ratio ofone to two. In the present method the oxide is typically a bismuth-basedoxide.

[0023] The present invention is also directed to a multifilamentaryoxide superconducting wire. The multifilamentary wire is obtained by themethod defined above. The multifilamentary wire includes a stabilizerand a plurality of filaments made of an oxide superconductor in contactwith the stabilizer, wherein its aspect ratio is one to two, theplurality of filaments are in the shape of a ribbon extending in thelongitudinal direction of the multifilamentary wire, the plurality offilaments are arranged substantially parallel to each other and arrangedin layers, the plurality of filaments have an aspect ratio greater thanthat of the multifilamentary wire, and the plurality of filamentsinclude a filament having a portion substantially not covered with thestabilizer.

[0024] In the present multifilamentary wire, preferably, the filamentshave an aspect ratio greater than two, more preferably have an aspectratio of at least ten. In the present multifilamentary wire the oxidesuperconductor is typically a bismuth-based oxide superconductor.

[0025] Furthermore the present invention is directed to a stranded oxidesuperconducting wire which is composed of a plurality of themultifilamentary oxide superconducting wires defined above that aretwined into the stranded wire. Each strand constituting the strandedwire has properties of the multifilamentary oxide superconducting wiredefined above.

[0026] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In the drawings:

[0028]FIG. 1 is a view for illustrating a technique of stackingtape-shaped wires to form a block having a low aspect ratio;

[0029]FIG. 2 is a schematic perspective view showing an example of themultifilamentary oxide superconducting wire according to the presentinvention;

[0030]FIG. 3 is a schematic, perspective view showing a typical shape ofthe filament of the multifilamentary wire shown in FIG. 2 in an enlargedmanner;

[0031]FIG. 4 is a schematic perspective view showing an example of thestranded wire according to the present invention;

[0032]FIG. 5 is a schematic cross section showing a cross section of thestranded wire shown in FIG. 4 in an enlarged manner;

[0033]FIG. 6 is a schematic, perspective view showing another example ofthe stranded wire according to the present invention;

[0034]FIG. 7 is a schematic, perspective view showing an example of thecutting step of the production method according to the presentinvention;

[0035]FIGS. 8A and 8B are schematic perspective views showing anotherexample of the cutting step of the production method according to thepresent invention;

[0036]FIG. 9 is a schematic, perspective view showing still anotherexample of the cutting step of the production method according to thepresent invention; and

[0037]FIG. 10 is a schematic, perspective view showing a process inwhich the cutting step and the twining step are carried outsimultaneously in a preferable mode of the present invention.

[0038] In the figures, reference numerals 10, 11 and 12 denote amultifilamentary oxide superconducting wire, a stabilizer and afilament, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039]FIG. 2 shows an example of the multifilamentary wire according tothe present invention. Multifilamentary oxide superconducting wire 10has stabilizer 11 and a plurality of filaments 12 in contact withstabilizer 11. Multifilamentary wire 10 is a monolithic conductor withthe matrix of stabilizer 11 and the plurality of filaments 12 combinedsubstantially inseparably. As shown in FIG. 2, in the three axes, the x,y and z axes, perpendicular to one another, the x axis corresponds tothe longitudinal direction of multifilamentary wire 10. In thissituation, as for the sizes of multifilamentary wire 10 in thedirections of the y and z axes, the longer size is defined as “width”and the shorter size is defined as “thickness.” In other words, in thepresent specification, as to the sizes of the wire in the twodirections, that are perpendicular to each other and both perpendicularto the longitudinal direction of the wire, the longer size is defined asa “width” and the shorter size is defined as a “thickness.” In thepresent invention, typically, the direction of the thickness of themultifilamentary wire corresponds to the direction of the thickness ofthe ribbon-shaped filament and the direction of the width of themultifilamentary wire corresponds to the direction of the width of theribbon-shaped filament, although the opposite correspondence may beapplied. In multifilamentary wire 10 of FIG. 2, the y axis correspondsto the direction of the width of the wire and the z axis corresponds tothe direction of the thickness of the wire. Multifilamentary wire 10 hasa ratio of width w to thickness t (w/t) in the range of 1 to 2. In thepresent specification, such a ratio of width to thickness are referredto as an aspect ratio. Each of filaments 12 is in the form of a ribbonextending in the longitudinal direction of multifilamentary wire 10.FIG. 3 shows a typical filament of multifilamentary wire 10. Thelongitudinal direction of filament 12 is typically parallel to that ofmultifilamentary wire 10. The cross section of filament 12 is arectangular shape, an elliptical shape or another shape similar thereto.Similarly to multifilamentary wire 10, filament 12 has its “thickness”and “width” in the directions perpendicular to its longitudinaldirection. As shown in FIG. 3, filament 12 has thickness t′significantly smaller than width w′ and the ratio of the width to thethickness (w′/t′) (the aspect ratio of filament 12) is higher than theaspect ratio of multifilamentary wire 10 (w/t). Typically, the filamenthas an aspect ratio greater than two. The filament may have an aspectratio (w′/t′) greater than seven, typically 7 to 20, preferably 10 to100, more preferably 10 to 50. In general, crystal grains of the oxidesuperconductor forming filament 12 are in the shape of a flake extendingin the longitudinal direction of filament 12 and typically the c crystalaxis of the oxide superconductor is strongly oriented in the directionof the thickness of filament 12. Typically, filament 12 has a layeredstructure with flake-shaped crystal grains overlapping each other andstrongly bonded together.

[0040] In multifilamentary wire 10 the plurality of filaments 12 arearranged substantially parallel to each other and arranged in layers.The plurality of filaments have their respective widths orientedsubstantially in the same direction. Preferably, the distances betweeneach pair of the adjacent filaments are substantially the same in thelayered direction. More specifically, it is preferable that theplurality of filaments are substantially uniformly distributed in thelayered direction. In multifilamentary wire 10 the plurality offilaments 12 include filaments 12 a, 12 b and 12 c having a portionwhich is substantially not covered with stabilizer 11. Morespecifically, filaments 12 a, 12 b and 12 c are partially exposed to theoutside of stabilizer 11. This exposure is attributed to the fact thatmultifilamentary wire 10 is obtained through the cutting step of theprocess according to the present invention. In the cutting step theoxide filaments are also cut so that the obtained multifilamentary wirehas such exposed filaments.

[0041] In the multifilamentary wire according to the present invention,one or more filaments may have a portion exposed to the outside of thestabilizer, and the number of such exposed filaments is not limited.According to the present invention, a multifilamentary wire may beprovided in which all of the filaments have a portion exposed to theoutside of the stabilizer. The number of the filaments having theportion not covered with the stabilizer may be not more than a half orpreferably not more than one third of the number of all the filaments.For a specific application, a coating may be provided on themultifilamentary wire so that the portion of the filament or filamentsexposed to the outside of the stabilizer can be covered with thecoating. Such a coating may preferably be made of an electricallyinsulating material such as enamel.

[0042] In the present invention, the oxide superconductor is typically abismuth-based oxide superconductor or a thallium-based oxidesuperconductor. In particular, the superconducting filaments arepreferably made of a bismuth-based, 2223-phase oxide superconductor suchas Bi₂Sr₂Ca₂Cu₃O_(10-X) and (Bi, Pb)₂Sr₂Ca₂Cu₃O_(10-X) or abismuth-based, 2212-phase oxide superconductor such as Bi₂Sr₂Ca₁Cu₂O_(8-X) and (Bi, Pb)₂Sr₂Ca₁Cu₂O_(8-X) Preferably, each filamenthas a single superconducting phase. A typical stabilizer is a noblemetal or a noble metal alloy, such as silver or a silver alloy. Thesilver alloy may include an Ag-Au alloy, an Ag—Mn alloy, an Ag—Al alloy,an Ag—Sb alloy, and an Ag—Ti alloy. In the multifilamentary wireaccording to the present invention, the ratio of the cross-sectionalarea of the stabilizer to the cross-sectional area of the filaments, socalled the silver ratio, is typically one to five, preferably two tofour.

[0043] In the multifilamentary wire according to the present invention,each filament may have a thickness of 1 μm to 100 μm, preferably 2 to 50μm, more preferably 5 to 40 μm. Each filament may have an aspect ratioof 7 to 200, preferably 10 to 100, more preferably 10 to 50. Thesuperconducting phase in the filaments may have the c-axis dominantlyoriented in the direction of the thickness of the filament. The c-axisof the crystal grains forming the filaments may form an angle of −20° to+20°, preferably −10° to +10°, more preferably −5° to +5° with thedirection of the thickness of each filament. The multifilamentary wireaccording to the present invention may have 2 to 10000 filaments,preferably 10 to 5000 filaments.

[0044] The multifilamentary wire according to the present invention mayhave a thickness of 0.1 mm to 2 mm, preferably 0.2 mm to 1 mm, and awidth of 0.2 mm to 2 mm, preferably 0.5 mm to 1 mm. The multifilamentarywire according to the present invention preferably has such a size thatit can constitute a stranded wire. The multifilamentary wire accordingto the present invention has an aspect ratio of one to two. Themultifilamentary wire having such an aspect ratio is suitable as astrand for constituting a stranded wire. Typically, in themultifilamentary wire according to the present invention, the crosssection perpendicular to the longitudinal direction thereof is arectangular shape, a square shape, a polygonal shape having at leastfive angles, or another shape similar thereto. The aspect ratio of eachfilament is greater than that of the multifilamentary wire. Typicallythe filament has an aspect ratio 1.5 to 200 times, preferably 5 to 100times greater than that of the multifilamentary wire. Themultifilamentary wire according to the present invention can have acritical current density (Jc) or a critical current per thecross-sectional area of the superconductor, of at least 10000 A/cm²,typically 10000 to 100000 A/cm².

[0045] Since the multifilamentary wire according to the presentinvention has ribbon-shaped filaments, it shows anisotropy as to thedirection of the magnetic field applied thereto. The multifilamentarywire according to the present invention, however, has a low aspect ratioand can have a Jc comparable to that of a typical, tape-shapedmultifilamentary oxide superconducting wire. Thus the present inventionis particularly used for a stranded wire.

[0046]FIGS. 4 and 5 show an example of the stranded wire according tothe present invention. A stranded oxide superconducting wire 30 iscomposed of ten multifilamentary oxide superconducting wires (strands)20 twined together at a given pitch. The pitch may be one centimeter ormore. Multifilamentary wire 20 has an aspect ratio of one.Multifilamentary wire 20 has a stabilizer 21 and a plurality offilaments 22 in contact with stabilizer 21. Multifilamentary wire 20 mayhave a structure similar to that of multifilamentary wire 10 as shownFIG. 2. In the plurality of filaments 22, some filaments have a portionthat is not covered with stabilizer 21. In stranded wire 30, thethickness or width directions of the filaments 22 are not the same butvary among the strands. In other words, the thickness or widthdirections of the strands are not the same but vary among the strands.These thickness or width directions preferably vary such that aminimized anisotropy can be obtained as to the magnetic field applied.Specifically, the arrangement or orientation of the strands may bechanged randomly or regularly so that the thickness or width directionsof the strands substantially uniformly vary in the range of 0° to 360°.The variation in the direction of the thickness or width, as describedabove, can provide the stranded wire with anisotropy reduced as to amagnetic field. The stranded structure can reduce current drift and acloss.

[0047]FIG. 6 shows another example of the stranded wire according to thepresent invention. A stranded, oxide superconducting wire 40 is formedof seven multifilamentary oxide superconducting wires (strands) 20twined together at a given pitch. The pitch may be one centimeter ormore. Multifilamentary wire 20 has an aspect ratio of one.Multifilamentary wire 20 has stabilizer 21 and a plurality of filaments22 in contact with stabilizer 21. Multifilamentary wire 20 may have astructure similar to that of multifilamentary wire 10 as shown in FIG.2. The plurality of filaments 22 include some filaments having a portionwhich is not covered with stabilizer 21.

[0048] The stranded wire according to the present invention may be aprimary stranded wire in which a plurality of strands are simply twinedtogether, a secondary stranded wire in which primary stranded wires arefurther twined together, or a tertiary or higher order stranded wirehaving a further stranded structure. The present invention may includeboth of a round stranded wire and a flat stranded wire. The strandedwire may be composed of two or more, preferably 7 to 49 strands. Thestranded wire may have a pitch of one centimeter or more, preferably twoto five centimeters. The pitch of the stranded wire may be five times ormore, preferably 10 to 20 times greater than the width of the strand.The stranded wire may or may not be twisted. In the stranded wireaccording to the present invention, each strand may be covered with anelectrically insulating material.

[0049] The above-described multifilamentary oxide superconducting wireis obtained by cutting a tape-shaped multifilamentary oxidesuperconducting wire in the process according to the present invention.The tape-shaped multifilamentary wire is prepared by a powder-in-tubeprocess. In the method according to the present invention, a tape-shapedmultifilamentary wire prepared by a typical powder-in-tube process canbe used. In a typical powder-in-tube process, oxide superconductorpowder or raw-material powder from which an oxide superconductor can beproduced is packed into a tube of a stabilizer, and then the tube filledwith the powder is subjected to plastic working and heat-treating toprovide a wire. The raw-material powder is typically prepared byblending at a given mixing ratio powdery oxides or carbonates of theelements constituting the superconductor, sintering the mixture and thengrinding the sintered material. The raw-material powder is packed into atube. The tube to be filled with the powder is typically made of silveror a silver alloy. The tube filled with the powder is subjected toplastic working, such as drawing, rolling, and pressing. To obtain amultifilamentary wire, typically, a plurality of round wires obtainedthrough drawing are packed into a tube and then drawn and rolled to givea tape-shaped multifilamentary wire. The tape-shaped multifilamentarywire thus obtained is heat-treated at an appropriate temperature toproduce a sintered body of the oxide superconductor. After the heattreatment, the multifilamentary wire is preferably rolled again andheat-treated for sintering.

[0050] In the method according to the present invention, the tape-shapedmultifilamentary wire which has been heat-treated for sintering ispreferably subjected to the cutting step. A preferable process from thelast rolling step that produces a tape-shaped multifilamentary wire maybe:

[0051] (A) rolling-sintering-rolling-cutting-sintering, or

[0052] (B) rolling-sintering-rolling-sintering-cutting.

[0053] In process A, a thermal treatment for sintering is followed byrolling and then followed by cutting and sintering. In process B, athermal treatment for sintering is followed by rolling and sintering,and then followed by cutting. In either process, the rolling step afterthe first sintering step can increase the density of the sintered body,resulting in improved Jc.

[0054] In the method according to the present invention, a cut or cutsare made in the tape-shaped, multifilamentary oxide superconducting wireprepared by the powder-in-tube process in its longitudinal direction toproduce a plurality of multifilamentary oxide superconducting wires eachhaving a width smaller than that of the tape-shaped wire. The presentinventor has found that even if the tape-shaped multifilamentary wire iscut in its longitudinal direction, the superconducting properties do notdeteriorate significantly. In the cutting step, press cutting, a wheelsaw with a hard material such as diamond, water-jet, or laser (e.g., alaser blade) may be used to cut the tape-shaped wire. The portion to becut may be subjected to any of these means, so that the original,tape-shaped wire is divided into wires having a width smaller than theoriginal. Preferably, the tape-shaped multifilamentary wire used has anaspect ratio of at least ten (10 to 30 for example), more preferably atleast 15 (15 to 25 for example). The tape-shaped multifilamentary wireused may have a thickness of 0.1 to 2 mm, preferably 0.2 to 1 mm, and awidth of 2 to 20 mm, preferably 4 to 10 mm. A specific manner of thecutting will be described below.

[0055] In an embodiment according to the present invention, atape-shaped multifilamentary wire 50 may be cut along the dotted linesas shown in FIG. 7. The dotted lines are all parallel to thelongitudinal direction of tape-shaped multifilamentary wire 50. By thecutting, tape-shaped multifilamentary wire 50 is divided into fournarrow multifilamentary wires each having an aspect ratio ofapproximately five. In another embodiment according to the presentinvention, as shown in FIG. 8A, cuts are made along the dotted lines toremove filament-free portions 60 a and 60 b at both ends of atape-shaped multifilamentary wire 60. The dotted lines are parallel tothe longitudinal direction of multifilamentary wire 60. Any of theabove-described means may be used for the cutting. As shown in FIG. 8B,cuts are then made in a tape-shaped multifilamentary wire 60′ along thedotted lines. The dotted lines are parallel to the longitudinaldirection of multifilamentary wire 60′. By the cutting, multifilamentarywire 60′ is divided into eight narrow multifilamentary wires having anaspect ratio of approximately two. Initially removing a filament-freeportion or portions can provide a more compact wire, having a reducedcross-sectional area per unit volume. The steps as shown in FIGS. 8A and8 B are advantageous, when the filament-free end portion or portionsoccupy a significant volume in the tape-shaped multifilamentary wire. Instill another embodiment according to the present invention, atape-shaped multifilamentary wire 70 may be cut along the dotted linesas shown in FIG. 9. By the cutting, tape-shaped multifilamentary wire 70is divided into 20 wires having an aspect ratio of approximately one.

[0056] In the cutting steps shown in FIGS. 7, 8A and 8B, and 9, somefilaments are cut and partially exposed to the outside of the obtainedmultifilamentary wires. Such wires may be used as they are, or a coatingmay be formed on them to cover the exposed filaments.

[0057] One advantage of the cutting step is that a narrow wire or wirescan be obtained through a relatively simple operation. For example, whena tape-shaped multifilamentary wire of 4 mm in width and 0.2 mm inthickness is cut into four aliquots, four wires each having a width of 1mm and a thickness of 0.2 mm, and thus having an aspect ratio of five,can readily be obtained. When the tape-shaped multifilamentary wire iscut into 20 aliquots, 20 wires each having a width of 0.2 mm and athickness of 0.2 mm, and thus having an aspect ratio of one, can readilybe obtained. In these examples, the original tape-shapedmultifilamentary wire can have substantially uniform superconductingproperties such as Jc and Ic over the cross section. Therefore, thewires obtained by cutting the original multifilamentary wire can alsohave substantially the same superconducting properties. Thus, narrowmultifilamentary wires having characteristics as required can beobtained through a relatively simple operation at a high yield. Incontrast, various problems arise if plastic working is employed toobtain a narrow wire. In particular, plastic working to reduce the widthto one millimeter or less tends to produce sausage-like filaments (i.e.,filaments being in the shape of wave rather than being linear in itslongitudinal direction) or tends to cut the filaments into fragments, sothat a wire having preferable superconducting properties can hardly beobtained. In contrast, the narrow wires obtained by cutting theoriginal, tape-shaped multifilamentary wire can substantially maintainthe same superconducting properties as the original.

[0058] After the cutting step, the obtained wires may be used as theyare, or they may be heat-treated for sintering. On the other hand, thewires obtained through the cutting step may be processed. The wireobtained through the cutting step typically has a rectangular or squarecross section, or other cross sections similar thereto. Such a crosssection can be changed by processing the wire to a more suitable shapefor producing a stranded wire. For example, the wire may be processed(e.g., chamfered) to have its corner or corners removed. The chamferedwire typically has a polygonal cross section, such as an octagon. Such aprocessed wire is more suitable for a stranded wire. After such aprocess, the wire may be heat-treated for sintering.

[0059] After the cutting step, the obtained wires can be twined togetherby a typical method. The stranding or twining pitch and the orientationof the strands (i.e., the orientation of the thickness or the width ofeach strand in the stranded wire) can be controlled typically bymodulating the revolution rate of the wire-supplying part. As shown inFIG. 10, wires may also be cut and twined together simultaneously. Aplurality of cutting means (e.g., a plurality of wheel saws) (not shown)are used to divide a tape-shaped multifilamentary wire 110 into narrowwires 115 and simultaneously a number of wires 115 are twined togethervia a rotating element 116 to provide a stranded wire 120. Rotatingelement 116 has holes 117, the number of which corresponds to that ofthe wires to be twined. Rotating element 116 rotating at a desired ratereceives wires 115 at holes 117, respectively, to provide stranded wire120. Alternatively, wires having a low aspect ratio resulting from thecutting step may be covered with an electrically insulating material andthen the covered wires may be twined together.

[0060] Hereinafter the present invention will more specifically bedescribed with reference to examples, although the present invention is,not limited to thereto.

EXAMPLE 1

[0061] Oxide powder and carbonate powder as starting materials weremixed to give a mixture having a Bi:Pb:Sr:Ca:Cu composition ratio of1.8:0.3:1.9:2.0:3.0. Heat treatment at 700° C. to 860° C. and grindingare performed on the mixture twice or more to prepare powder forpacking. The obtained powder was packed into a silver pipe having aninner diameter of 10 mm and an outer diameter of 12 mm and then the pipefilled with the powder was drawn. The resulting wire having a diameterof 1.1 mm was cut to prepare 61 wires for engagement. The 61 wires forengagement were packed into a silver pipe having an inner diameter of 10mm and an outer diameter of 12 mm, and then the pipe filled with thewires were drawn and rolled to give a tape-shaped wire having a width of3.8 mm and a thickness of 0.25 mm. The tape-shaped wire was heated atapproximately 850° C. under the atmosphere for sintering. After cooled,the tape-shaped wire was rolled to have a width of 4.2 mm and athickness of 0.20 mm. It was heat-treated again at approximately 850° C.under the atmosphere and cooled to provide a tape-shapedmultifilamentary wire of 10 m in length. This tape-shapedmultifilamentary wire was 4.2 mm wide and 0.2 mm thick and had acritical current (Ic) of 60 A at the liquid nitrogen temperature withouta magnetic field applied. Thus, it was calculated that the tape had acritical current value per unit area (overall Jc) of approximately7.14×10³A/cm² (60 A/(0.42 cm×0.02 cm)).

[0062] From the tape-shaped multifilamentary wire of ten meters long,filament-free, 0.1 mm-wide portions at both ends were cut off with adiamond cutter. A wheel saw was used for the diamond cutter. Thetape-shaped multifilamentary wire having the both end portions removedhad a critical current of 58 A. Its overall Jc was approximately7.25×10³ A/cm² (58 A/(0.4 cm×0.02 cm)), which is a slightly larger valuethan the initial Jc. Then, the tape-shaped multifilamentary wire wasfurther cut with the diamond cutter and divided into four wires of onemillimeter in width. The obtained four wires had critical currents of 15A, 14 A, 15 A and 14 A respectively. The wires having an Ic of 15 A hadan overall Jc of 7.5×10³ A/cm²(15 A/(0.1 cm×0.02 cm)) and the wireshaving an Ic of 14 A had an overall Jc of 7.0×10³ A/cm² (14 A/(0.1cm×0.02 cm)). The four wires obtained all had their respective Jc closeto that of the original tape-shaped multifilamentary wire. It has beenunderstood that multifilamentary wires having a low aspect ratio and arelatively high Jc can be produced through the cutting step.

[0063] Other tape-shaped multifilamentary wires were prepared for thecutting step. Those had the same cross-sectional area but differentaspect ratios. Table 1 shows the relationship between the aspect ratiosand the Ic at to the prepared tape-shaped wires. It is understood fromthe table that tape-shaped multifilamentary wires having an aspect ratioof at least ten are more preferable for the cutting step. TABLE 1 Width(mm) 4 3.5 3 2 1 Thickness (mm) 0.2 0.228 0.265 0.4 0.8 Aspect Ratio 2016 11 5 1.25 Critical Current (A) 70 65 55 22 9

EXAMPLE 2

[0064] Except for the shape of the finally obtained wire, a processsimilar to that employed in Example 1 was employed to prepare a 61filament, tape-shaped wire of one meter in length, 10 mm in width and 1mm in thickness, whose aspect ratio is ten. The obtainedmultifilamentary wire had an Ic of 600 A. This multifilamentary wire wasdivided by means of a diamond cutter into ten aliquots to give ten wireseach having a length of one millimeter, a width of 1 mm and a thicknessof 1 mm, and having an aspect ratio of one. Each wire had a square crosssection. Each wire whose aspect ratio is one had substantially the sameoverall Jc as the original, tape-shaped wire. The obtained ten wireswere manually twined together to provide a stranded wire. The obtainedstranded wire had an Ic of approximately 580 A at the liquid nitrogentemperature without a magnetic field applied.

[0065] According to the present invention, multifilamentary oxidesuperconducting wires having a relatively high Jc and a low aspect ratiocan be provided. Such multifilamentary wires are more suitable forapplications such as stranded wires and solenoid coils. According to thepresent invention, stranded wires having a relatively high Jc can alsobe provided. In such stranded wires, ac loss can be small, so that theycan be more suitable for energy transfer.

[0066] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. A method of producing a multifilamentary oxidesuperconducting wire, comprising the step of making a cut or cuts in atape-shaped multifilamentary oxide superconducting wire prepared by apowder-in-tube process, along a longitudinal direction thereof, so thata plurality of multifilamentary oxide superconducting wires each havinga width smaller than that of said tape-shaped wire are obtained fromsaid tape-shaped wire.
 2. The method of claim 1 , wherein saidtape-shaped wire has an aspect ratio of at least ten.
 3. The method ofclaim 1 , wherein said multifilamentary oxide superconducting wiressmaller in width have an aspect ratio of one to two.
 4. The method ofclaim 2 , wherein said multifilamentary oxide superconducting wiressmaller in width have an aspect ratio of one to two.
 5. The method ofclaim 1 , wherein said oxide is a bismuth-based oxide.
 6. The method ofclaim 2 , wherein said oxide is a bismuth-based oxide.
 7. The method ofclaim 3 , wherein said oxide is a bismuth-based oxide.
 8. A method ofproducing a stranded oxide superconducting wire, comprising the step oftwining a required number of multifilamentary oxide superconductingwires obtained by the method of claim 1 .
 9. The method of claim 8 ,wherein said tape-shaped wire has an aspect ratio of at least ten. 10.The method of claim 8 , wherein said multifilamentary oxidesuperconducting wires smaller in width have an aspect ratio of one totwo.
 11. The method of claim 8 , wherein said oxide is a bismuth-basedoxide.
 12. A multifilamentary oxide superconducting wire comprising astabilizer and a plurality of filaments made of an oxide superconductorin contact with said stabilizer, wherein said multifilamentary oxidesuperconducting wire has an aspect ratio of one to two, said pluralityof filaments are in the shape of a ribbon extending in a longitudinaldirection of said multifilamentary wire, said plurality of filaments arearranged substantially parallel to each other and in layers, saidplurality of filaments have an aspect ratio greater than that of saidmultifilamentary wire, and said plurality of filaments include afilament or filaments having a portion substantially not covered withsaid stabilizer.
 13. The multifilamentary oxide superconducting wire ofclaim 12 , wherein said filaments have an aspect ratio greater than two.14. The multifilamentary oxide superconducting wire of claim 12 ,wherein said oxide superconductor is a bismuth-based oxidesuperconductor.
 15. The multifilamentary oxide superconducting wire ofclaim 13 , wherein said oxide superconductor is a bismuth-based oxidesuperconductor.
 16. A stranded oxide superconducting wire, which iscomposed of a plurality of the multifilamentary oxide superconductingwire of claim 12 twined together.
 17. The stranded oxide superconductingwire of claim 16 , wherein said filaments have an aspect ratio greaterthan two.
 18. The stranded oxide superconducting wire of claim 16 ,wherein said oxide superconductor is a bismuth-based oxidesuperconductor.
 19. The stranded oxide superconducting wire of claim 17, wherein said oxide superconductor is a bismuth-based oxidesuperconductor.